'Isshouni' and 'Isshoni' may sound similar, but they aren't, and they bring completely different meaning!
If you are saying "Isshouni hirugohan o tabemasen ka." to a person, it means that for your entire life, you are going to eat only lunch with that person! And it is impossible to eat only lunch, unless you fast for your entire life. X'D
Also, 'Isshouni' has a longer sound compared to 'Isshoni', so make sure you know the difference when you hear that word, too! Who knows if your crush is actually asking you out for the entire life or something? ;)
I'll end my post early, and will return once in a while to prepare for my final exams! I hope I would score well! Please wish me luck! ^ u ^
Until then.

0 Comments